PMD Testing

If you have recently looked at the specifications for
polarization-mode-dispersion (PMD) test equipment, you
might have noticed that some vendors quote PMD uncertainty
in “weak-mode coupling” condition, while others talk about
“strong-mode coupling” condition. What do these two
concepts refer to? This article will explain the two types
of mode coupling and show that they are as different as
apples and oranges.

Let’s begin with a basic fact: light
is made of two perpendicular
polarizations, commonly named
electric and magnetic fields (or
waves) that travel into the fiber.
Add to that fact the reality that
fiber is not perfect, which is
caused by the core roundness,
pureness, etc., or the local
imperfections or stresses that
can create variation in the index
of refraction (which can be seen
as the density of the glass).
Therefore, on a given length of
fiber, one path will on average
have less stress than another,
enabling light to travel faster;
this is often referred to as the
fast axis as opposed to the slow
axis; note that these two axes are
perpendicular.

Each fiber section can have a
different fast and slow axis and
the length of these sections can
greatly vary depending on the
type and age of the fiber. Every
time a section change occurs,
energy transfers from one mode
(fast or slow) to something
else, depending on the section
that follows (known as mode
coupling), and the fibers in which
this occurs are usually referred to
as strong-mode coupling.
In some fibers, this randomness
is eliminated by imposing a faster
fiber path during production; for
example, this can be achieved
by creating index of refraction
variations that are much greater
than those that occur due to
imperfections (referred to as
polarization-maintaining fibers
(PMF)). Several different PMF
designs are used and most
work by inducing stress in the
core via a non-circular cladding
cross-section or rods of other
material included within the
cladding. Since there is only one
section with always the same fast
and slow axis, these fibers are
also referred to as weak-mode
coupling.

Upon closer examination of the light pulse, composed of two perpendicular polarizations, it should be noted that as this light enters a fiber that has two different propagation speeds: one of the polarizations will go faster than the other, splitting the logical “1” into two subcomponents, as illustrated in figure 3.

WEAK-MODE COUPLING

In a weak-mode coupling fiber, such as PMF, PMD is measured as the delay at the arrival point between the two subcomponents (see figure 4). Since the delay is fixed and engineered in the fiber, it will not change with time and external factors. Wavelength change or input state of polarization change will not modify the delay between the two propagation axes, making PMD measurements easily accessible while providing a high degree of accuracy.

STRONG-MODE COUPLING

The first segment of the fiber acts as a local PMF section—it splits the pulse in two, as seen in
Figure 5.

In this case, since no speed path was engineered, there is a disturbance/imperfection/impurity a few meters after this section, and a mode-coupling change occurs, which is typical of any telecom fiber.

The two output pulses in figure 5 are once again split in two and lead to the result illustrated in figure 6.
As the length of the fiber continues, this process is repeated a number of times, which results in a pulse that is much broader with a random distribution, as illustrated in figure 7.

PMD is defined from the root
mean square (RMS) or average
width of the spread. Each
wavelength will see different
fast and slow axis, which will
change with variations in
temperature, time and the
input state of polarization. This
results in significant variations
in the overall output shape of
the pulse over time. The model
defined in the standards calls for
infinite coupling with a perfectly
distributed pulse shape at the
end, but the reality is that telecom
fibers can be made from anything
from two to an infinite amount of
sections, and they are completely
unpredictable.

Some test equipment makes this
assumption (infinite coupling);
i.e., instruments based on the
fixed-analyzer method, as well
as those based on the traditional
interferometric method (TINTY),
which can lead to potentially large
errors. Even for test equipment
that goes past these assumptions
(i.e., the general interferometric
method and the SOP scrambling
analysis method), measuring
a value that is an average or
the RMS of several varying and
random parts is much more
challenging, yet this challenge
needs to be addressed since it is a
real-world issue.

CONCLUSION

All telecom fibers—whether
standard fibers, non-zero
dispersion-shifted fibers
(NZDSFs), bend-insensitive fibers
etc.—are all strongmode coupling
fibers. Their type, length, age and
environment dictate the amount
and distribution of the coupling
sections and ratios, making it
more challenging to test the fiber
for PMD. Some test equipment
vendors base their accuracy
specifications on weak-mode
coupling, a condition never seen
in network scenarios and that
is extremely easy to measure
and qualify. An accuracy value
based on a test scenario that
will never be seen in the field
cannot guarantee anything on
real-world fibers. Some other test
equipment vendors tackle the
challenge and specify accuracy
with strong-mode coupling. This
accuracy is often not as good as
the values quoted on weak-mode
coupling, but it is a usable number
and certainly has a true accuracy
rate that is much higher than
those specifying only weak-mode
coupling values, which are in
reality focusing on a specification
rather than an application.